1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries.
2. Description of Related Art
In recent years, lithium-ion secondary batteries, which are one of several types of non-aqueous electrolyte secondary batteries, have been used as a power source for various mobile devices. The power consumption of such mobile devices has been increasing as the number of functions of mobile devices has increased, and there is a corresponding demand for lithium-ion secondary batteries having higher energy densities.
For that reason, lithium-containing oxide produced by ion exchanging sodium-containing oxide has been studied as a candidate for a next-generation high-capacity positive electrode active material.
The positive electrode active material LiCoO2, which has already been in commercial use, is characterized by having an O3 structure belonging to the space group R-3m. On the other hand, the LiCoO2 prepared by ion exchanging is characterized by having an O2 structure, and it typically belongs to the space group P63mc. The O3 structure is a crystal structure in which a lithium atom exists at the center of the oxygen octahedron and three kinds of stacks comprising a layer including oxygen and a layer including a transition metal oxide MO6 exist per unit cell. On the other hand, the O2 structure is a crystal structure in which a lithium atom exists at the center of the oxygen octahedron, as in the O3 structure, and two kinds of stacks comprising a layer including oxygen and a layer including a transition metal oxide MO6 exist per unit cell.
When the LiCoO2 having an O3 structure is charged to 4.6 V (vs. Li/Li+), about 50% of the lithium in the LiCoO2 is extracted, and consequently, the crystal structure thereof deteriorates, resulting in degradation of the reversibility of lithium insertion and deinsertion. On the other hand, the LiCoO2 having an O2 structure is capable of being charged and discharged even when about 80% of the lithium in the LiCoO2 is extracted (see Solid State Ionics, 144 (2001) 263), so it is one of the most promising candidates for the next generation high-capacity positive electrode active material.
However, the LiCoO2 having an O2 structure is difficult to produce. The LiCoO2 having an O2 structure can be obtained by preparing Na0.7CoO2 having a P2 structure and ion exchanging the sodium with lithium. However, when the temperature during the ion exchanging exceeds 150° C., LiCoO2 having an O3 structure results, whereas the temperature is too low, the source material not having been ion-exchanged remains.
Another problem with the LiCoO2 having an O2 structure and the LiCoO2 having an O3 structure has been that they show poor charge-discharge cycle performance when the end-of-charge potential is set at 4.6 V (vs. Li/Li+) or higher. The cause is believed to be that an irreversible change in the crystal structure occurs during the process of the lithium deinsertion from the crystal, and therefore it is no longer possible to maintain the initial crystal structure.
Japanese Published Unexamined Patent Application No. 2009-32681 (Patent Document 1) proposes adding lithium to the sodium oxide used as the starting material. The publication shows that the capacity can be increased because the positive electrode active material contains a complementary structure of Li2MnO3 in addition to the main structure. It is believed that the interaction between the main structure and the complementary structure contributes to an increase of the capacity density of the positive electrode active material.
However, the just-described positive electrode active material shows an average discharge potential of only about 3.6 V when the potential is in the range of from 2.0 V to 5.0 V (vs. Li/Li+), so the material is not desirable as a next generation high-capacity positive electrode material.